Impeller of Cooling Fan

ABSTRACT

An impeller of a cooling fan includes a metal base, a shaft and a plurality of plastic blades. The metal base includes a shaft-coupling portion, a first assembling portion apart from the shaft-coupling portion, and a flat portion between the shaft-coupling portion and the first assembling portion, wherein the flat portion oppositely has a first flat surface and a second flat surface, and a permanent magnet is mounted on the first flat surface. The shaft has a fixing end coupled with the shaft-coupling portion of the metal base. Each of the plastic blades has a second assembling portion coupled with the first assembling portion of the metal base.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an impeller of cooling fan and, more particularly, to an impeller suitable for thin cooling fan.

2. Description of the Related Art

In the current trend of miniaturization for electrical devices, cooling fans disposed inside miniaturized electrical devices to drive air currents for cooling have to be smaller than conventional fans. In this condition, the size of an impeller in a cooling fan becomes a critical feature of whether the cooling fan is suitable to be applied in a miniaturized electrical device.

Referring to FIGS. 1 and 2, an invention of Taiwan Patent No. 1293106, entitled as “Thin-Type Fan,” is shown. The invention is about a conventional cooling fan 8 including a base 81, an impeller 82, a magnetic plate 83 and a shaft 84. The base 81 has a central hole 811 and a coil unit 812. The impeller 82 has a plurality of bent blades 821. The magnetic plate 83 is mounted on the bottom of the impeller 82. The shaft 84 rotatably inserts into the central hole 811 of the base 81 with one end and fixedly couples with the impeller 82 with the other end. With the above arrangement, the impeller 82 of this conventional cooling fan 8 is thin and suitable to be applied in kinds of miniaturized electrical devices.

However, the impeller 82 of cooling fan 8 is conventionally made by punching a metal plate into the shape of an impeller with the outer periphery of the metal plate being bent to form the blades 821. Although the impeller 82 made of the metal plate is thin, the manufacturing process of the impeller 82 is complex and a large amount of metal is needed, such that manufacturing cost of this impeller 82 is large. Besides, the choices in shape and inclination of the blades 821 are few since the blades 821 are formed by punching, so that it is impossible to greatly enhance the air-driving performance by modifying the shape or inclination of the blades 821. Furthermore, the metal impeller 82 is much heavier than others made of plastic, and this may result in high loading in weight and further affect rotational performance. Finally, it is difficult to bend the blades 821 formed by punching into identical shapes, and thus air-driving and rotational balance performances are lowered too.

Another example, referring to FIG. 3, is shown by the contents of Taiwan Patent No. M350746, entitled as “Thin-Type Fan Rotor.” This patent discloses a thin impeller that is a common impeller design currently available in the market. It discloses that a rotor 9 has a metal housing 91, a shaft 92 coupled to a center of the metal housing 91, a metal blade frame 93 extending outwards from the periphery of the metal housing 91 in a radial direction of the rotor 9, and a plastic blade portion 94 fixed with the metal blade frame 93. Specifically, since the rotor 9 is mainly constructed by the metal housing 91 and the metal blade frame 93, a desirable structural strength of the rotor 9 is provided. Moreover, due to the plastic blade portion 94 for air driving, which is easy to be uniformly formed by injection molding, the rotor 9 has advantages such as easy fabrication, low manufacturing cost and desirable air-driving performance. Accordingly, the rotor 9 is thin as well as strong.

However, it is not flat from a center part of the metal housing 91 by which the metal housing 91 couples with the shaft 92 to the outer periphery of the metal blade frame 93 where the plastic blade portion 94 couples with. Specifically, the periphery of the metal housing 91 has a hub portion 95 axially bulging relative to the metal blade frame 93, which results in a large axial thickness of the rotor 9. Therefore, due to the metal housing 91 having an increased thickness, the rotor 9 inside an electrical device may become a stumbling block of axial miniaturization of the electrical device. As a result, it is necessary to improve the conventional rotors.

SUMMARY OF THE INVENTION

It is therefore the objective of this invention to provide an impeller having a metal base with compact axial size, so as to render a cooling fan with the impeller suitable for use in miniaturized electrical devices.

It is another objective of this invention to provide an impeller constructed by the metal base and a plurality of plastic blades coupling with the metal base, so that the impeller is thin but strong.

One embodiment of the invention discloses an impeller of a cooling fan, which includes a metal base, a shaft and a plurality of plastic blades. The metal base includes a shaft-coupling portion, a first assembling portion apart from the shaft-coupling portion, and a flat portion between the shaft-coupling portion and the first assembling portion, wherein the flat portion oppositely has a first flat surface and a second flat surface, and a permanent magnet is mounted on the first flat surface. The shaft has a fixing end coupled with the shaft-coupling portion of the metal base. Each of the plastic blades has a second assembling portion coupled with the first assembling portion of the metal base.

In a preferred form shown, the second assembling portions jointly form an enclosing ring coupled with the first assembling portion of the metal base.

In the preferred form shown, the first assembling portion of the metal base has a plurality of through holes, and the second assembling portions of the plastic blades fill up the through holes.

In the preferred form shown, the through holes are arranged in a circular route adjacent to a periphery of the metal base, and any adjacent two of the through holes are spaced out a predetermined distance apart.

In the preferred form shown, the first assembling portion of the metal base has a plurality of saw teeth at a periphery of the metal base, and the second assembling portions of the plastic blades couple with the saw teeth at the periphery of the metal base.

In the preferred form shown, the first assembling portion of the metal base has an inclined section forming the periphery of the metal base, and the second assembling portions of the plastic blades couple with the inclined section of the first assembling portion of the metal base.

In the preferred form shown, the second assembling portions couple with the inclined section of the first assembling portion by covering both of first and second surfaces of the inclined section, wherein the first surface connects with and extends from the first flat surface of the flat portion, and the second surface connects with and extends from the second flat surface of the flat portion.

In the preferred form shown, a through hole is disposed in the inclined section and extends from a first surface of the inclined section to a second surface of the inclined section, and the second assembling portions fill the through hole, with the first surface connecting with and extending from the first flat surface of the flat portion, and with the second surface connecting with and extending from the second flat surface of the flat portion.

In the preferred form shown, the first assembling portion of the metal base has a stair section, and the second assembling portions of the plastic blades couple with the stair section of the first assembling portion of the metal base.

In the preferred form shown, the second assembling portions couple with the stair section of the first assembling portion by covering both of first and second surfaces of the stair section, wherein the first surface connects with and extends from the first flat surface of the flat portion, and the second surface connects with and extends from the second flat surface of the flat portion.

In the preferred form shown, the stair section includes a radial extension and an axial extension linking the flat portion and the radial extension.

In the preferred form shown, through holes are disposed in at least one of the axial and radial extensions of the stair section and extend from a first surface of the stair section to a second surface of the stair section, and the second assembling portions fill the through hole, with the first surface connecting with and extending from the first flat surface of the flat portion, and with the second surface connecting with and extending from the second flat surface of the flat portion.

In the preferred form shown, the flat portion and the first assembling portion are coplanar.

In the preferred form shown, the flat portion and the first assembling portion are non-coplanar.

In the preferred form shown, in an axial direction of the shaft-coupling portion of the metal base, the flat portion has a first axial thickness, the first assembling portion has a second axial thickness, and a ratio of the second axial thickness to the first axial thickness falls in a range between 1.1 and 8.5, and the ratio of the second axial thickness to the first axial thickness preferably falls in a range between 1.1 and 4.

In the preferred form shown, the first and second flat surfaces are uncovered or covered with a plastic or rustproof layer.

In the preferred form shown, an axial end face of the fixing end is at a same level of or is lower than the second flat surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a perspective and exploded view of a conventional thin-type fan.

FIG. 2 is a cross-sectional view of the conventional thin-type fan.

FIG. 3 is a cross-sectional view of a conventional thin-type fan rotor.

FIG. 4 is a perspective view of an impeller of a cooling fan according to a first embodiment of the invention.

FIG. 5 is a cross-sectional view of the impeller of the first embodiment of the invention.

FIG. 6 is a top view of the impeller of the first embodiment of the invention.

FIG. 7 is a detailed and cross-sectional view of a combination example of a metal base and a shaft of the said impeller.

FIG. 8 is a detailed and cross-sectional view of another combination example of the metal base and the shaft of the said impeller.

FIG. 9 is a top view of an impeller of a cooling fan according to a second embodiment of the invention.

FIG. 10 is a cross-sectional view of an impeller of a cooling fan according to a third embodiment of the invention.

FIG. 11 is a cross-sectional view of an impeller of a cooling fan according to a fourth embodiment of the invention.

FIG. 12 is a cross-sectional view of the impeller of the fourth embodiment of the invention with through holes disposed in a radial extension of a stair section.

FIG. 13 is a cross-sectional view of the impeller of the fourth embodiment of the invention with through holes disposed in both of axial and radial extensions of a stair section.

In the various figures of the drawings, the same numerals designate the same or similar parts. Furthermore, when the term “first,” “second,” “third,” “fourth,” “top,” “lower,” and similar terms are used hereinafter, it should be understood that these terms refer only to the structure shown in the drawings as it would appear to a person viewing the drawings, and are utilized only to facilitate describing the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 4, 5 and 6, a first embodiment of an impeller of a cooling fan is shown, which includes a metal base 1, a shaft 2 coupled with a central part of the metal base 1, and a plurality of plastic blades 3 coupled with a periphery of the metal base 1.

The metal base 1 has a shaft-coupling portion 11 and a first assembling portion 12. The shaft-coupling portion 11 is formed in the central part of the metal base 1 and is a means for coupling with the shaft 2. In this embodiment, the shaft-coupling portion 11 is a hole for partially receiving the shaft 2. The first assembling portion 12 is apart from the shaft-coupling portion 11 and is a means for the plurality of plastic blades 3 to couple with the metal base 1, wherein the first assembling portion 12 can be selected from various structures for the injection-formed plastic blades 3 to mount on, which is discussed in detail later. The metal base 1 further includes a flat portion 13 having no axial curve and between the shaft-coupling portion 11 and the first assembling portion 12, with the flat portion 13 preferably linking the shaft-coupling portion 11 and the first assembling portion 12. Specifically, the flat portion 13 oppositely includes a first flat surface 131 and a second flat surface 132, with a permanent magnet 14 mounted on the first flat surface 131. The first and second flat surfaces 131, 132 may be uncovered or covered with a plastic or rustproof layer. In addition, the metal base 1 is preferably made of magnetic-conducting material, so that the metal base 1 can also effectively provide flux-guiding and shielding functions when the permanent magnet 14 is set. The metal base 1 is characterized in the flatness of the flat portion 13, wherein the first and second flat surfaces 131, 132 do not have any element similar to a hub, so that the axial size of the metal base 1 is small and thus this metal base 1 is favorable for application in miniaturized electrical devices.

The shaft 2 has a fixing end 21 coupled with the shaft-coupling portion 11. The way to connect the fixing end 21 of the shaft 2 with the shaft-coupling portion 11 of the metal base 1 can be selected from injection molding, fastening, screwing, welding, close fitting, adhesion or the like. Specifically, FIG. 5 shows that the fixing end 21 couples with the shaft-coupling portion 11 by welding, close fitting or adhesion; FIG. 7 shows that the fixing end 21 couples with the shaft-coupling portion 11 by a fastening ring; and FIG. 8 shows that the fixing end 21 couples with the shaft-coupling portion 11 by an injection-molded member. Preferably, an axial end face 211 of the fixing end 21 is at the same level of or is lower than the second flat surface 132, so that an axial size of the impeller may not increase by the shaft 2. As a result, the impeller for a cooling fan is suitable to be applied in a miniaturized electrical device.

Each of the plastic blades 3 has a second assembling portion 31 and an air-driving portion 32. The second assembling portion 31 couples with the first assembling portion 12 of the metal base 1, wherein the second assembling portion 31 is a means for coupling with the first assembling portion 12 by injection molding. Specifically, the second assembling portions 31 of the plural plastic blades 3 can be separately formed and jointly surround the metal base 1 as shown in FIGS. 5 and 6. However, the said second assembling portions 31 also can be integrally connected in a ring shape circularly enclosing the metal base 1 as shown in FIGS. 9 through 13, so as to ensure that the inclinations of all the plastic blades 3 relative to the central part of the metal base 1 are the same. Besides, with the ring jointly formed by the said second assembling portions 31, there are more choices in shape and inclination for the plastic blades 3 to be arranged so as to provide larger air current and better rotational balance.

The embodiments of the first assembling portion 12 of the metal base 1 and the second assembling portion 31 of the plastic blades 3 can be divided into several types discussed as following:

Particularly, in this first embodiment, referring to FIGS. 5 and 6 again, the first assembling portion 12 of the metal base 1 has a plurality of through holes 121 arranged in a circular route adjacent to the periphery of the metal base 1, wherein any adjacent two of the through holes 121 are spaced out a predetermined distance apart. The second assembling portions 31 of the plastic blades 3 fill up the through holes 121 of the first assembling portion 12 by injection-molding, wherein the second assembling portion 31 of each plastic blade 3 is in the form of a wrapping part 311 coupling with inner walls defining the through holes 121. On the other hand, if the second assembling portions 31 are integrally connected to form the ring shape enclosing the metal base 1 circularly, the ring serves as the second assembling portions 31 and couples with all the through holes 121 by injection molding. As a result, the plurality of plastic blades 3 can fixedly couple with the metal base 1 and get rid of radial loose and disengagement from the metal base 1.

Referring to FIG. 9, a second embodiment of the impeller of a cooling fan is disclosed. In comparison with the first embodiment, the first assembling portion 12 of the metal base 1 provides a plurality of saw teeth 122 at the periphery of the metal base 1, and ends of the plurality of plastic blades 3, which couple with the periphery of the metal base 1, jointly form an enclosing ring 312 serving as the second assembling portions 31. As a result, the plurality of plastic blades 3 can steadily couple with the metal base 1 and get rid of radial loose and disengagement from the metal base 1. Furthermore, compared with FIGS. 5 and 6, with the saw teeth 122 and the enclosing ring 312, the combination between the metal base 1 and the plastic blades 3 can be apparently improved.

Referring to FIG. 10, a third embodiment of the impeller of a cooling fan is disclosed. In comparison with the first and second embodiments, the first assembling portion 12 of the metal base 1 has an inclined section 123 forming the periphery of the metal base 1. The inclined section 123 is inclined relative to the first and second flat surface 131, 132 of the flat portion 13, such as bent toward the first flat surface 131 or toward the second flat surface 132. Additionally, ends of the plurality of plastic blades 3, which couple with the periphery of the metal base 1, jointly form an enclosing ring 313 serving as the second assembling portions 31. The injection-molded enclosing ring 313 couples with the inclined section 123 of the first assembling portion 12 by covering both of first and second surfaces of the inclined section 123, wherein the said first surface connects with and extends from the first flat surface 131, and the said second surface connects with and extends from the second flat surface 132. Furthermore, there can be a through hole in the inclined section 123 extending from the first surface to the second surface, so that the enclosing ring 313 can fill the through hole to achieve an enhanced assembling performance between the plastic blades 3 and the metal base 1. Accordingly, with the arrangement in this embodiment, the plurality of plastic blades 3 can also fixedly couple with the metal base 1 and get rid of radial loose and disengagement from the metal base 1. Additionally, in comparison with the first and second embodiments shown by FIGS. 5, 6 and 9, since the inclined section 123 is totally enclosed inside the enclosing ring 313, the coupling area between the plastic blades 3 and the metal base 1 is enlarged while the axial size of the metal base 1 is maintained. As a result, the coupling strength between the plastic blades 3 and the metal base 1 is enhanced in this embodiment.

Referring to FIG. 11, a fourth embodiment of the impeller of a cooling fan is disclosed. In comparison with the first, second and third embodiments, the first assembling portion 12 of the metal base 1 has a stair section 124 forming the periphery of the metal base 1, wherein the stair section 124 includes a radial extension and an axial extension linking the flat portion 13 and the radial extension. Similarly, ends of the plurality of plastic blades 3, which couple with the periphery of the metal base 1 and serve as the second assembling portions 31, jointly form an enclosing ring 314. The injection-molded enclosing ring 314 couples with the stair section 124 of the first assembling portion 12 by covering both of first and second surfaces of the stair section 124, wherein the said first surface connects with and extends from the first flat surface 131, and the said second surface connects with and extends from the second flat surface 132. Furthermore, there can be through holes 125 in at least one of the axial and radial extensions of the stair section 124 extending from the first surface to the second surface, so that the enclosing ring 314 can fill the through holes 125 to achieve an enhanced assembling performance between the plastic blades 3 and the metal base 1. Specifically, referring to FIG. 12, one of the through holes 125 is disposed in the axial extension of the stair section 124; and, referring to FIG. 13, two of the through holes 125 are respectively disposed in the axial and radial extensions of the stair section 124. Accordingly, with the arrangement in this embodiment, the plurality of plastic blades 3 can also fixedly couple with the metal base 1 and get rid of radial loose and disengagement from the metal base 1. Additionally, in comparison with the first and second embodiments shown by FIGS. 5, 6 and 9, since the stair section 124 is totally enclosed inside the enclosing ring 314, the coupling area between the plastic blades 3 and the metal base 1 is enlarged while the axial size of the metal base 1 is maintained. The coupling strength between the plastic blades 3 and the metal base 1 is strongly enhanced by increasing the coupling area of the plastic blades 3 and the metal base 1. Besides, in comparison with the third embodiment shown by FIG. 10, the coupling area between the plastic blades 3 and the metal base 1 is also relatively enlarged. As a result, the coupling strength between the plastic blades 3 and the metal base 1 is further enhanced in this embodiment.

The above-disclosed embodiments can be divided into two types. The first type includes the first and second embodiments shown in FIGS. 5, 6 and 9, wherein the flat portion 13 and the first assembling portion 12 are coplanar. Specifically, in an axial direction of the shaft-coupling portion 11 of the metal base 1, the flat portion 13 has a first axial thickness “H1,” the first assembling portion 12 has a second axial thickness “H2,” and a ratio of the second axial thickness “H2” to the first axial thickness “H1” is 1. The second type includes the third and fourth embodiments shown in FIGS. 10 and 11, wherein the flat portion 13 and the first assembling portion 12 are non-coplanar, and the coupling strength between the plastic blades 3 and the metal base 1 is improved. Particularly, based on that the first assembling portion 12 axially extends within an axial extending range of the plastic blades 3, a ratio of the second axial thickness “H2” to the first axial thickness “H1” falls in a range between 1.1 and 8.5, and it is preferably between 1.1 and 4. Therefore, the coupling strength between the plastic blades 3 and the metal base 1 is further enhanced in this embodiment.

When the proposed impeller is in use, it can be served as an impeller in an axial fan, a blower, or an advection fan. Taking a blower as an example, the proposed impeller is rotatably mounted in a fan frame and driven by a motor via conventional ways. Therefore, the proposed impeller is suitable for use in kinds of miniaturized electrical devices to achieve cooling purpose in operation.

In sum, the main characteristic of the metal base 1 of the disclosed impeller is that the metal base 1 includes a shaft-coupling portion 11, a first assembling portion 12 apart from the shaft-coupling portion 1, and a flat portion 13 between the shaft-coupling portion 11 and the first assembling portion 12, wherein the flat portion 13 oppositely has a first flat surface 131 and a second flat surface 132. There is no hub portion axially bulging relative to the first flat surface 131 and the second flat surface 132. As a result, the impeller has a compact size in axial direction and is suitable for application in miniaturized electrical devices.

Furthermore, the disclosed impeller mainly includes the metal base 1 and the plastic blades 3, wherein the metal base 1 is formed by a metal plate with low thickness and high structural strength while the plastic blades 3 provide desirable elasticity and low manufacturing cost. Besides, base on the disclosed impeller mainly includes the metal base 1 and the plastic blades 3, the impeller is thin as well strong.

Although the invention has been described in detail with reference to its presently preferable embodiments, it will be understood by one of ordinary skill in the art that various modifications can be made without departing from the spirit and the scope of the invention, as set forth in the appended claims. 

What is claimed is:
 1. An impeller of a cooling fan, comprising: a metal base comprising a shaft-coupling portion, a first assembling portion apart from the shaft-coupling portion, and a flat portion between the shaft-coupling portion and the first assembling portion, wherein the flat portion oppositely has a first flat surface and a second flat surface, and a permanent magnet is mounted on the first flat surface; a shaft having a fixing end coupled with the shaft-coupling portion of the metal base; and a plurality of plastic blades, wherein each plastic blade has a second assembling portion coupled with the first assembling portion of the metal base.
 2. The impeller of the cooling fan as claimed in claim 1, wherein the second assembling portions jointly form an enclosing ring coupled with the first assembling portion of the metal base.
 3. The impeller of the cooling fan as claimed in claim 1, wherein the first assembling portion of the metal base has a plurality of through holes, and the second assembling portions of the plastic blades fill up the through holes.
 4. The impeller of the cooling fan as claimed in claim 3, wherein the through holes are arranged in a circular route adjacent to a periphery of the metal base, and any adjacent two of the through holes are spaced out a predetermined distance apart.
 5. The impeller of the cooling fan as claimed in claim 1, wherein the first assembling portion of the metal base has a plurality of saw teeth at a periphery of the metal base, and the second assembling portions of the plastic blades couple with the saw teeth at the periphery of the metal base.
 6. The impeller of the cooling fan as claimed in claim 1, wherein the first assembling portion of the metal base has an inclined section forming the periphery of the metal base, and the second assembling portions of the plastic blades couple with the inclined section of the first assembling portion of the metal base.
 7. The impeller of the cooling fan as claimed in claim 6, wherein the second assembling portions couple with the inclined section of the first assembling portion by covering both of first and second surfaces of the inclined section, wherein the first surface connects with and extends from the first flat surface of the flat portion, and the second surface connects with and extends from the second flat surface of the flat portion.
 8. The impeller of the cooling fan as claimed in claim 6, wherein a through hole is disposed in the inclined section and extends from a first surface of the inclined section to a second surface of the inclined section, and the second assembling portions fill the through hole, with the first surface connecting with and extending from the first flat surface of the flat portion, and with the second surface connecting with and extending from the second flat surface of the flat portion.
 9. The impeller of the cooling fan as claimed in claim 1, wherein the first assembling portion of the metal base has a stair section, and the second assembling portions of the plastic blades couple with the stair section of the first assembling portion of the metal base.
 10. The impeller of the cooling fan as claimed in claim 9, wherein the second assembling portions couple with the stair section of the first assembling portion by covering both of first and second surfaces of the stair section, wherein the first surface connects with and extends from the first flat surface of the flat portion, and the second surface connects with and extends from the second flat surface of the flat portion.
 11. The impeller of the cooling fan as claimed in claim 9, wherein the stair section includes a radial extension extending in a radial direction of the metal base and an axial extension extending in an axial direction of the metal base, with the axial extension linking the flat portion and the radial extension.
 12. The impeller of the cooling fan as claimed in claim 9, wherein through holes are disposed in at least one of the axial and radial extensions of the stair section and extend from a first surface of the stair section to a second surface of the stair section, and the second assembling portions fill the through hole, with the first surface connecting with and extending from the first flat surface of the flat portion, and with the second surface connecting with and extending from the second flat surface of the flat portion.
 13. The impeller of the cooling fan as claimed in claim 1, wherein the flat portion and the first assembling portion are coplanar.
 14. The impeller of the cooling fan as claimed in claim 1, wherein the flat portion and the first assembling portion are non-coplanar.
 15. The impeller of the cooling fan as claimed in claim 14, wherein, in an axial direction of the shaft-coupling portion of the metal base, the flat portion has a first axial thickness, the first assembling portion has a second axial thickness, and a ratio of the second axial thickness to the first axial thickness falls in a range between 1.1 and 8.5.
 16. The impeller of the cooling fan as claimed in claim 15, wherein the ratio of the second axial thickness to the first axial thickness falls in a range between 1.1 and
 4. 17. The impeller of the cooling fan as claimed in claim 9, wherein the flat portion and the first assembling portion are non-coplanar.
 18. The impeller of the cooling fan as claimed in claim 17, wherein, in an axial direction of the shaft-coupling portion of the metal base, the flat portion has a first axial thickness, the first assembling portion has a second axial thickness, and a ratio of the second axial thickness to the first axial thickness falls in a range between 1.1 and 8.5.
 19. The impeller of the cooling fan as claimed in claim 18, wherein the ratio of the second axial thickness to the first axial thickness falls in a range between 1.1 and
 4. 20. The impeller of the cooling fan as claimed in claim 1, wherein the first and second flat surfaces are uncovered.
 21. The impeller of the cooling fan as claimed in claim 1, wherein the first and second flat surfaces are covered with a plastic or rustproof layer.
 22. The impeller of the cooling fan as claimed in claim 1, wherein an axial end face of the fixing end is at a same level of or is lower than the second flat surface. 